In order to achieve a high process quality in laser material processing systems, compliance with the process- and laser parameters within narrow limits is necessary. The process windows are very small, especially in highly dynamic processing systems. In order to ensure the processing quality, a regular and accurate monitoring of the parameters of the laser beam is therefore necessary. This is particularly true for systems in which the laser beam can be freely positioned and moved by means of a scanning device in an at least two-dimensional working region, for example by deflecting the beam over movable mirrors and subsequent scanner optics. Such remote laser processing equipment has a variety of applications, such as labeling, welding, cutting, and the like. A relatively new application is Selective Laser Melting (SLM). In this method, complex three-dimensional objects can be produced by local melting or sintering of successive thin layers of a powdered material. Since the production time of the articles in such a process is naturally long, a shortening of the production time is desirable. For this purpose, the laser beam must be moved faster, for which purpose higher laser powers are required in order to melt the powder in a shorter time. In SLM systems, therefore, laser beams with a very small focus and relatively high power are used, i.e. radiation sources with high brilliance.
Many different methods are known for determining geometric parameters of a laser beam. A principal possibility for measurement is, for example, to direct the beam directly or indirectly to a spatially resolving sensor, for example to a CCD camera, and in this way to determine the intensity distribution in the cross-section of the beam. From this data, further information such as the beam diameter or the position of the beam can also be derived or calculated. In principle, the problem of attenuating the laser beam before the beam strikes the sensor without changing the beam in its properties must be solved here.
Publications JP01107990A and JP2008264789A, for example, disclose apparatus with spatially resolving sensors, which are designed for the geometrical calibration of remote systems. In these apparatus, the light scattered by a substrate arranged in the working field is imaged onto a camera by means of a lens. From this, the position of the beam in the working field can be determined and compared with the target position. However, such apparatus are not suitable for measuring the focus diameter or the intensity distribution in the focus of the laser beam because of the achievable spatial resolution.
Apparatus from the available technology are also known in which a spatially resolving sensor is arranged in a beam path, which is coupled out by means of a beam splitter before the focusing of the laser beam. A beam component reflected back into the focusing of the laser beam is then used for the beam diagnosis. The reflection of the beam takes place, for example, on a boundary surface of the imaging optical system, typically on the last boundary surface of the focusing lens, or on a downstream protective glass. A general method of this type is disclosed in DE102007053632. An apparatus of a similar type, which is also designed for use in scanner optics, is shown in DE102011054941. A disadvantage of the apparatus known from the available technology is that the back-image by the focusing lens is partially loaded with considerable imaging errors, since the back-reflected beam basically has different dimensions and waist positions and the correction of the focusing lens is not simultaneously designed for the imaging of reflections with different dimensions and waist positions. This disadvantageous effect is reinforced even more through protective glass designed as concave mirror for back reflection, which is proposed in DE102011054941. The known systems are therefore not suitable for precise beam diagnosis. At best, with the known systems, relative changes in the focusing lens can be recognized by a measurement in new condition and a target/actual comparison.
Another basic possibility for beam diagnosis is to sample the beam. Many variants are known for this purpose. Sampling can take place with a near-point-shaped probe, for example a measuring aperture or a measuring needle whose opening is small compared to the beam diameter. In this case, the beam must be scanned in several passes with marginally offset lines. In this way, the two-dimensional intensity distribution of the beam can be determined.
However, a sampling can also take place with a line probe, for example with a slot or with a slot aperture. In the case of linear sampling, the beam intensity is already integrated in one direction. The advantage is that the beam diameter can be determined with a single scanning pass.
A relative movement between beam and sampling probe is necessary for sampling. In the case of a stationary beam the probe must be necessarily moved. Numerous apparatus, which are not specifically intended for use in remote systems, are known for this measuring principle. Apparatus with quasi-punctiform sampling systems are known, for example, from DE19909595 and EP0461730. Publications U.S. Pat. No. 5,078,491 and JP62024117A disclose, for example, apparatus with linear sampling systems. A further apparatus with essentially linear sampling is known from WO89/50196, in which, for example, an optical fiber is proposed for sampling the beam.
In sampling systems, the achievable resolution is always limited by the size of the sampling aperture. In order to determine the desired parameter, for example the beam diameter or the beam profile, a computational expansion with the sampling function is also required. Therefore, complex operations and calculations must be carried out in order to obtain the desired parameters. A further disadvantage is that sampling takes a long time and complicated apparatus are required for the precise control and movement of the sampling probe.
In the case of apparatus for beam diagnosis in remote laser processing systems, however, there is the specific feature that the laser beam can be deflected in two dimensions by means of a scanning device so that the beam can be freely positioned in a two-dimensional or sometimes three-dimensional working space. In this case, it is also possible that the sampling probe is stationary and the beam is moved by means of the scanning device via the sampling probe.
According to the latter principle, for example the method disclosed in DE2005038587 works. There, a measuring system is proposed in which a laser beam can be moved by means of a deflection system in a specifiable pattern via a detector arrangement. A similar method according to this principle is shown in DE102011006553. A method for determining the focus position or the beam profile of a laser beam by means of a scanning system is specified there, in which a pinhole aperture with a subsequent detector is arranged at several measuring points in the working space of the laser beam. At each of the measuring points for an x-y-focus position or beam profile measurement, the laser beam is moved by means of the scanner optics to an x-y grid over the measuring hole of the pinhole aperture.
Another similar apparatus of the aforementioned type is disclosed in U.S. Pat. No. 6,501,061. The laser beam is scanned via an aperture and the scanner can be position-calibrated by comparing the scanner position data at the time of the laser beam detection.
The apparatus known from the available technology, which use scanning optics for sampling, have the following disadvantages: On one hand the beam data can usually only be recorded at individual positions, on the other hand the attainable spatial resolution is directly related to the precision with which the laser beam can be moved or positioned by means of the scanner optics. The precise measurement of the beam profile of a finely-focused laser beam of a low-order mode is generally not possible in this way.
A fundamental problem with the measurement of laser radiation is furthermore the high power density in the beam, which can quickly lead to the destruction of the sensors used. Semiconductor sensors, such as photodiodes or CCD cameras, are particularly sensitive in this respect, which is why the laser beam must first be attenuated by many orders of magnitude. However, the elements used for attenuation often change the beam such that it is problematic to recalculate to the exact beam parameters of the original beam. Optical elements can cause thermally induced changes in the image due to low absorption of laser radiation. If such effects are to be detected in the laser processing optics by means of a beam diagnostic apparatus, then it is disadvantageous, if attenuating elements increase this effect. On the other hand, the scanning systems can have an advantage since the beam is moved relative to the scanner and therefore the action takes place only briefly. However, many passes are usually required for scanning, so that an action often occurs one behind the other. In addition, current high-brilliancy laser sources have such high power densities in the beam that it is also generally necessary for scanning systems to direct only a fraction of the beam intensity to the respectively used sensor.
The apparatus and methods known from the available technology therefore have considerable disadvantages, both with regard to the attainable accuracy and spatial resolution as well as with respect to the compatibility with laser beams of high power.